The present invention relates to a fuel control apparatus for an internal combustion engine for controlling the amount of fuel suppled to the engine.
FIG. 8 illustrates an example of a conventional fuel control apparatus for an engine as described in Japanese Patent Laid-Open No. 59-103965. The apparatus illustrated is used for a four-cylinder gasoline engine 1 of an automotive vehicle. The engine 1 includes four cylinders 1a (only one is illustrated for the sake of simplicity in explanation), an intake pipe or manifold 3, a throttle valve 4 in the intake manifold 3, an injector 5 for each cylinder 1a for injecting fuel thereto, an intake valve 6 for each cylinder 1a, an exhaust manifold 7, an exhaust valve 8 for each cylinder 1a, a spark plug 9 for each cylinder 1a, a distributor 10, pressure sensors 11 each sensing the absolute internal pressure in a corresponding cylinder 1a, and a signal generator 12 for sensing the rotational angle of the engine. The signal generator 12 includes a top dead center sensor 12a which takes out a signal from a gear operatively connected with a camshaft, which rotates in synchrony with the rotation of the engine crankshaft, for sensing when the piston in a first cylinder is at top dead center, and a crank angle sensor 12b which takes out a signal from the gear for sensing the crank angle or position of the crankshaft. An intake air temperature sensor 14 is mounted in the intake manifold 3 for sensing the temperature of intake air sucked into the cylinders 1a. An engine coolant temperature sensor 15 is mounted on the side wall of each cylinder 1a for sensing the temperature of an engine coolant. An air-fuel ratio sensor 16 is mounted in the exhaust manifold 7 for sensing the air-fuel ratio of a mixture supplied to the cylinders 1a based on the concentration of certain component such as oxygen in the exhaust gas. An ignition coil and igniter combination 17 supplies a high voltage to the distributor 10 causing each spark plug 9 to generate a spark. An electronic control unit 100 (hereinafter referred to as an ECU) successively calculates an appropriate amount of fuel to be injected into each cylinder 1a and an appropriate ignition timing based on the output signals from the sensors 11 through 16, and controls the injector 5 and the ignition coil and igniter combination 17 on the basis of the fuel injection amount and the ignition timing thus calculated. The ECU 100 may be a digital computer including a CPU in the form of a microprocessor, a ROM, a RAM and an I/O interface having an input port and an output port, all of which are interconnected to each other through a bidirectional bus.
FIG. 9 shows various control timings of the various processings or operations of the above-described conventional apparatus. As shown in FIG. 9, the intake valve 6 for the first cylinder 1a closes at a crank angle of 30 degrees after bottom dead center (BDC2), and after the lapse of 10 degrees from the closing of the intake valve 6 or at a crank angle of 40 degrees after bottom dead center, the ECU 100 measures the internal pressure in each cylinder 1a. When the crankshaft of the engine 1 rotates to reach top dead center (TDC2), the ECU 100 calculates the number of revolutions per minute of the engine 1, sets an appropriate ignition timing for this cylinder 1a and starts the power supply to the ignition coil of the ignition coil and igniter combination 17. The above processing steps are repeated so that four ignitions and two injections per two revolutions of the crankshaft are carried out.
The operation of the conventional fuel control apparatus will be described in detail while referring to FIG. 10 which is a flow chart of a main routine. First, in Step M1, the ECU 100 is powered for initialization while setting therein necessary data for prescribed calculations and clearing the unillustrated RAM in the ECU 100. In Step M2, the output voltage of the coolant temperature sensor 15 is converted from analog to digital form and read out as a digital value. Then in Step M3, a coolant temperature correction coefficient is calculated on the basis of the presently measured temperature of the engine coolant and data of previously measured engine coolant temperatures prestored in the unillustrated ROM in the ECU 100. In Step M4, the output voltage of the intake air temperature sensor 14 is converted from analog to digital form and read out as a digital value. Subsequently in Step M5, an intake air temperature correction coefficient is calculated on the basis of the presently measured intake air temperature and data of previously measured intake air temperatures prestored in the ROM. In Step M6, the number of revolutions per minute of the engine 1, which is calculated in a crank angle timed interrupt process and stored in the RAM, is read out therefrom. The engine rpm is calculated by measuring the time between successive half revolutions of the engine crankshaft. Then in Step M7, cylinder pressure, which is measured in a crank angle timed interrupt process and stored in the RAM, is read out. In Step M8, basic fuel injection time is calculated on the basis of the presently measured cylinder pressure and the presently calculated engine rpm while looking at a two-dimensional map stored in the ROM in which the fuel injection time is plotted as a function of cylinder pressure and engine rpm, and the basic fuel injection time thus calculated is then multiplied by the above calculated engine coolant temperature correction coefficient and the above calculated intake air temperature correction coefficient to provide a corrected fuel injection time, which is stored in the RAM. Thereafter in Step M9, based on the presently measured cylinder pressure and the presently measured engine rpm, a basic ignition timing is calculated while looking at a two-dimensional map stored in the ROM in which the ignition timing is plotted as a function of the cylinder pressure and the engine rpm, and the basic ignition timing thus obtained is added by the above calculated engine coolant temperature correction coefficient to obtain a corrected ignition timing, which is stored in the RAM. In this manner, on the basis of the corrected fuel injection time and the corrected ignition timing thus set in the RAM, the injectors 5 and the ignition coil of the ignition coil and igniter combination 7 are driven or energized at respective timings as illustrated in the timing chart of FIG. 9.
Although in the above-described conventional fuel control apparatus, the calculation processes are based on the cylinder pressure measured at a point of time during each compression stroke, the amount of fuel to be injected can be calculated on the basis of a difference P between two cylinder pressures measured at two specific crank positions, as described in Japanese Patent Laid-Open No. 59-221433.
With the conventional fuel control apparatus as described above, the cylinder pressure on each compression stroke varies from one cycle to another. To compensate for such variations in the cylinder pressure, averaging the measured cylinder pressures is required, but this results in a delay in the processing time required for calculating the amount of intake air and the amount of fuel injection. Particularly, the response in fuel control during engine acceleration is impaired, reducing the output torque of the engine.